The European Patent Application EP 1 471 176 A1 describes a method whereby fibers are passed through a spray mist of fluid droplets. The fluid droplets are smaller than 20 μm in size. This method provides for electrically charging the fluid droplets in a separation process utilizing the Lenard effect (“Lenard charging effect”). Once charged, the liquid droplets are passed, together with the fibers, through an electric field. In this method, statistically distributed fluid droplets having different polarities are formed. Thus, both positively, as well as negatively charged fluid droplets are formed in this case, and the distribution thereof can only be controlled with difficulty. Moreover, the polarity of the electric charge of the fluid droplets is dependent on the size thereof. The mechanical spraying divides the fluid droplets into large and small fluid droplets, each having a different electric charge.
Filter elements of the type described at the outset are likewise already known from the related art. In particular, it is known to use different methods to charge nonwoven fabrics made of microfibers. The European Patent Application EP 0 845 058 A1 discusses a method for producing filter elements having a very high filtration efficiency. These filter elements are distinguished by a very high filtration efficiency against fine aerosols.
The method known from the European Patent Application EP 0 845 058 A1 provides for applying water droplets as charge carriers to a finished nonwoven fabric. The water droplets, which are applied at the same time, have different charges. To improve the electric charge capacity, an additive is admixed with the polymer from which the fibers of the nonwoven are produced.
The inherent disadvantage of this method is that the water droplets are sprayed onto the already finished nonwoven fabric. Employing this method causes filter effects to occur which prevent a continuous distribution of the charges within the nonwoven fabric. The denser the nonwoven fabric that is used is, the higher the water pressures are that are to be used to homogeneously distribute charges within the nonwoven fabric. However, these high water pressures can negatively alter or even destroy the structure of the nonwoven fabric.
Moreover, a drying process is required following the spray application of large quantities of water. During the drying process, the nonwoven fabric is thermally treated, which can result in a reduction of the electric charge. Therefore, it is generally preferred that the drying process take place at low temperatures in a vacuum. This entails a substantial outlay for equipment and involves considerable costs.